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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="container.extended_functionality"></a><a class="link" href="extended_functionality.html" title="Extended functionality: Basic extensions">Extended functionality:
    Basic extensions</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.default_initialialization">Default
      initialization for vector-like containers</a></span></dt>
<dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.ordered_range_insertion">Ordered
      range insertion for associative containers (<span class="emphasis"><em>ordered_unique_range</em></span>,
      <span class="emphasis"><em>ordered_range</em></span>) </a></span></dt>
<dt><span class="section"><a href="extended_functionality.html#container.extended_functionality.constant_time_range_splice">Constant-time
      range splice for <code class="computeroutput"><span class="special">(</span><span class="identifier">s</span><span class="special">)</span><span class="identifier">list</span></code></a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="container.extended_functionality.default_initialialization"></a><a class="link" href="extended_functionality.html#container.extended_functionality.default_initialialization" title="Default initialization for vector-like containers">Default
      initialization for vector-like containers</a>
</h3></div></div></div>
<p>
        STL and most other containers value initialize new elements in common operations
        like <code class="computeroutput"><span class="identifier">vector</span><span class="special">::</span><span class="identifier">resize</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">)</span></code> or <code class="computeroutput"><span class="keyword">explicit</span>
        <span class="identifier">vector</span><span class="special">::</span><span class="identifier">vector</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">)</span></code>.
      </p>
<p>
        In some performance-sensitive environments, where vectors are used as a replacement
        for variable-size buffers for file or network operations, <a href="http://en.cppreference.com/w/cpp/language/value_initialization" target="_top">value
        initialization</a> is a cost that is not negligible as elements are going
        to be overwritten by an external source shortly after new elements are added
        to the container.
      </p>
<p>
        <span class="bold"><strong>Boost.Container</strong></span> offers two new members for
        <code class="computeroutput"><span class="identifier">vector</span></code>, <code class="computeroutput"><span class="identifier">static_vector</span></code>
        and <code class="computeroutput"><span class="identifier">stable_vector</span></code>: <code class="computeroutput"><span class="keyword">explicit</span> <span class="identifier">container</span><span class="special">::</span><span class="identifier">container</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">,</span> <span class="identifier">default_init_t</span><span class="special">)</span></code> and <code class="computeroutput"><span class="identifier">container</span><span class="special">::</span><span class="identifier">resize</span><span class="special">(</span><span class="identifier">size_type</span> <span class="identifier">n</span><span class="special">,</span> <span class="identifier">default_init_t</span><span class="special">)</span></code>, where new elements are constructed using
        <a href="http://en.cppreference.com/w/cpp/language/default_initialization" target="_top">default
        initialization</a>.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="container.extended_functionality.ordered_range_insertion"></a><a class="link" href="extended_functionality.html#container.extended_functionality.ordered_range_insertion" title="Ordered range insertion for associative containers (ordered_unique_range, ordered_range)">Ordered
      range insertion for associative containers (<span class="emphasis"><em>ordered_unique_range</em></span>,
      <span class="emphasis"><em>ordered_range</em></span>) </a>
</h3></div></div></div>
<p>
        When filling associative containers big performance gains can be achieved
        if the input range to be inserted is guaranteed by the user to be ordered
        according to the predicate. This can happen when inserting values from a
        <code class="computeroutput"><span class="identifier">set</span></code> to a <code class="computeroutput"><span class="identifier">multiset</span></code>
        or between different associative container families (<code class="computeroutput"><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span><span class="special">/</span><span class="identifier">map</span></code>
        vs. <code class="computeroutput"><span class="identifier">flat_</span><span class="special">[</span><span class="identifier">multi</span><span class="special">]</span><span class="identifier">set</span><span class="special">/</span><span class="identifier">map</span></code>).
      </p>
<p>
        <span class="bold"><strong>Boost.Container</strong></span> has some overloads for constructors
        and insertions taking an <code class="computeroutput"><span class="identifier">ordered_unique_range_t</span></code>
        or an <code class="computeroutput"><span class="identifier">ordered_range_t</span></code> tag
        parameters as the first argument. When an <code class="computeroutput"><span class="identifier">ordered_unique_range_t</span></code>
        overload is used, the user notifies the container that the input range is
        ordered according to the container predicate and has no duplicates. When
        an <code class="computeroutput"><span class="identifier">ordered_range_t</span></code> overload
        is used, the user notifies the container that the input range is ordered
        according to the container predicate but it might have duplicates. With this
        information, the container can avoid multiple predicate calls and improve
        insertion times.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="container.extended_functionality.constant_time_range_splice"></a><a class="link" href="extended_functionality.html#container.extended_functionality.constant_time_range_splice" title="Constant-time range splice for (s)list">Constant-time
      range splice for <code class="computeroutput"><span class="special">(</span><span class="identifier">s</span><span class="special">)</span><span class="identifier">list</span></code></a>
</h3></div></div></div>
<p>
        In the first C++ standard <code class="computeroutput"><span class="identifier">list</span><span class="special">::</span><span class="identifier">size</span><span class="special">()</span></code> was not required to be constant-time, and
        that caused some controversy in the C++ community. Quoting Howard Hinnant's
        <a href="http://howardhinnant.github.io/On_list_size.html" target="_top"><span class="emphasis"><em>On
        List Size</em></span></a> paper:
      </p>
<div class="blockquote"><blockquote class="blockquote">
<p>
          <span class="emphasis"><em>There is a considerable debate on whether <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">size</span><span class="special">()</span></code> should be O(1) or O(N). The usual argument
          notes that it is a tradeoff with:</em></span>
        </p>
<p>
          <code class="computeroutput"><span class="identifier">splice</span><span class="special">(</span><span class="identifier">iterator</span> <span class="identifier">position</span><span class="special">,</span> <span class="identifier">list</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">iterator</span>
          <span class="identifier">first</span><span class="special">,</span>
          <span class="identifier">iterator</span> <span class="identifier">last</span><span class="special">);</span></code>
        </p>
<p>
          <span class="emphasis"><em>If size() is O(1) and this != &amp;x, then this method must perform
          a linear operation so that it can adjust the size member in each list</em></span>
        </p>
</blockquote></div>
<p>
        C++11 definitely required <code class="computeroutput"><span class="identifier">size</span><span class="special">()</span></code> to be O(1), so range splice became O(N).
        However, Howard Hinnant's paper proposed a new <code class="computeroutput"><span class="identifier">splice</span></code>
        overload so that even O(1) <code class="computeroutput"><span class="identifier">list</span><span class="special">:</span><span class="identifier">size</span><span class="special">()</span></code>
        implementations could achieve O(1) range splice when the range size was known
        to the caller:
      </p>
<div class="blockquote"><blockquote class="blockquote">
<p>
          <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">splice</span><span class="special">(</span><span class="identifier">iterator</span> <span class="identifier">position</span><span class="special">,</span> <span class="identifier">list</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">iterator</span>
          <span class="identifier">first</span><span class="special">,</span>
          <span class="identifier">iterator</span> <span class="identifier">last</span><span class="special">,</span> <span class="identifier">size_type</span>
          <span class="identifier">n</span><span class="special">);</span></code>
        </p>
<p>
          <span class="bold"><strong>Effects</strong></span>: Inserts elements in the range
          <code class="computeroutput"><span class="special">[</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">)</span></code> before position and removes the elements
          from x.
        </p>
<p>
          <span class="bold"><strong>Requires</strong></span>: <code class="computeroutput"><span class="special">[</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">)</span></code>
          is a valid range in x. The result is undefined if position is an iterator
          in the range <code class="computeroutput"><span class="special">[</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">)</span></code>. Invalidates only the iterators and references
          to the spliced elements. <code class="computeroutput"><span class="identifier">n</span> <span class="special">==</span> <span class="identifier">distance</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">)</span></code>.
        </p>
<p>
          <span class="bold"><strong>Throws</strong></span>: Nothing.
        </p>
<p>
          <span class="bold"><strong>Complexity</strong></span>: Constant time.
        </p>
</blockquote></div>
<p>
        This new splice signature allows the client to pass the distance of the input
        range in. This information is often available at the call site. If it is
        passed in, then the operation is constant time, even with an O(1) size.
      </p>
<p>
        <span class="bold"><strong>Boost.Container</strong></span> implements this overload
        for <code class="computeroutput"><span class="identifier">list</span></code> and a modified version
        of it for <code class="computeroutput"><span class="identifier">slist</span></code> (as <code class="computeroutput"><span class="identifier">slist</span><span class="special">::</span><span class="identifier">size</span><span class="special">()</span></code>
        is also <code class="computeroutput"><span class="identifier">O</span><span class="special">(</span><span class="number">1</span><span class="special">)</span></code>).
      </p>
</div>
</div>
<div class="copyright-footer">Copyright © 2009-2018 Ion Gaztanaga<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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